(1) Field of the Invention
The present invention relates to an optical recording medium comprising a recording film of a metal formed on a substrate. In this optical recording medium, the metal film is irradiated with laser beams to melt or evaporate the film and form small pits or cause deformation, whereby information can be recorded at a high density, and reproduction is performed by utilizing changes in the light reflectance or transmission in the recorded areas.
(2) Description of the Related Art
A process is known in which laser beams are converged on a recording film formed on a disk-shaped transparent substrate of a glass or plastic material and a physical deformation, a phase change or the like occurs in the laser beam-converged portion of the recording film, and information is recorded by utilizing this deformation or change. If a high output laser such as an argon laser is used as the laser beam source for recording, a film of a high-melting-point metal having a high oxidation resistance can be used as the recording film, and the range of selection of the recording film is expanded. But the size of the recording and reproducing apparatus, per se, is increased and an expensive modulator must be provided for modulation of the laser beams, and thus this process is not suitable for practical application. Accordingly, semiconductor lasers are mainly used at present, but semiconductor lasers have problems in that the wavelength of laser beam is limited to that in the near-infrared region and the output is low. Accordingly, a recording medium of a heat mode optical recording medium used for recording information according to the above-mentioned process must have a high absorption to light in the near-infrared region.
The heat mode optical recording medium most frequently used at present is the type wherein pits are formed in a laser beam-converged portion and information is recorded by utilizing these pits. In order for this optical recording medium to show good recording characteristics, the material of the recording film must have a low melting point, the material must have a low heat conductivity, the material must have an appropriate light reflectance, and pits having a good shape must be formed. As the conventional material of the optical recording film satisfying these requirements, there are known films prepared by vacuum deposition or sputtering of metals such as tellurium, bismuth, selenium and tin, alloys composed mainly of these metals and composites of these metals or alloys with organic materials or oxides, and films formed by spin-coating of organic dyes having a light absorption band in the near-infrared region.
Especially, a film of tin formed by vacuum deposition or sputtering has a low melting point and high light absorption and reflectance in the near-infrared region, and therefore, a recording film of the pit-forming type having a high recording sensitivity and a high contrast ratio can be obtained from tin. Moreover, this recording film is advantageous over other metal type optical recording materials in that the material is cheap and has a low toxicity.
Although the tin film has the above-mentioned characteristics, a film of tin formed by vacuum deposition or sputtering has a cohesion structure and the surface is roughened, and therefore, if recording and reproduction are carried out on this recording film, the noise due to the surface condition of the film is large and a good C/N ratio cannot be obtained. Moreover, the tin film is often deteriorated by oxidation, and if the tin film is allowed to stand in air for a long time, a reduction of the reflectance and a formation of pinholes occur.
As the means for obviating the foregoing defects without degradation of the high sensitivity possessed by the tin film, there can be mentioned a method in which another metal is added to tin, a method in which a compound of tin is used, and a method in which fine particles of tin are dispersed in a tin compound. Since films prepared according to these methods, which comprise tin as the main component, are composed of fine particles of tin, the above-mentioned noise due to the surface condition of the film is drastically reduced. Furthermore, according to these methods, the oxidation resistance of the film is increased, and reduction of the reflectance of the recording film or degradation of the recording-reproducing characteristics, caused when the film is stored at room temperature for a long time, can be controlled to a high degree.
As the substrate acting as a support for the recording film, a glass substrate, a plastic substrate composed of polymethyl methacrylate, polycarbonate, an epoxy resin or the like, and a substrate which is formed by coating a recording film-forming side of a substrate as mentioned above with an ultraviolet ray-curable resin or the like for formation of a guide groove or the like, are often used.
Especially, the plastic substrate is frequently used because the molding is easily performed, little cracking occurs handling is easy, and the heat conductivity is low.
If the above-mentioned film comprising tin as the main component is allowed to stand in a high-temperature and high-humidity atmosphere for a long time, because of an adhesion of water drops or other foreign substances, pinholes are formed by pitting and the bit error rate is increased, with the result that a problem arises with respect to the reliability of the medium.
Moreover, if this film comprising tin as the main component is formed on a plastic substrate such as polymethyl methacrylate or polycarbonate and recording is conducted on this recording medium, the formation of pits is incomplete and the film material is not completely removed from the laser beam-irradiated area but a slight amount thereby remains in the interiors of pits, as shown in FIG. 2. Therefore, the C/N ratio upon reproduction is unsatisfactory and the bit error rate is high.